Libyan revolutionary forces are engaged in fierce fighting with Gaddafi loyalists in one of the biggest assaults yet in the battle for the deposed despot's hometown of Sirte.

Nato warplanes circled overhead as fighters battled on the streets and snipers shot at the advancing forces of the National Transitional Council (NTC).

NTC forces earlier swept into the town with heavy guns mounted on around 100 vehicles but they met strong resistance as they pushed to within less than half a mile (kilometer) of the loyalist fighters' main position around the Ouagadougou convention centre and Green Square.

The interim government's troops had to seek cover after coming under heavy bombardment from fighters loyal to Gaddafi. Two revolutionary fighters were killed and another three wounded in the exchanges.

The NTC forces pushed deep into Sirte on Friday, taking most of it street by street. At least 15 people were killed and more than 180 injured, doctors said. Thousands of civilians have left but many remain behind.

Gaddafi loyalists also remain entrenched in the city of Bani Walid, but the NTC intends to declare the country liberated once Sirte has fallen because they will then have control over all ports and harbours.

Mustafa Abdul-Jalil, the head of the governing National Transitional Council, said the battle was "ferocious".

"Our fighters today are still dealing with the snipers positioned on the high buildings and we sustained heavy casualties," he said.

Suleiman Ali, commander for revolutionary forces, said loyalist forces have been driven away from Ibn Sina Hospital where hundreds of civilians have sought refuge from the fighting.

A military spokesman in Tripoli, Abdel-Rahman Busin, said he expected the city to be declared free in the next 24 hours.

"They've pretty much taken the city and it's just a few pockets of resistance," he said. But he added that snipers remained a major threat.

On Saturday, British defence secretary Liam Fox pledged to continue Nato airstrikes, even after Sirte's fall, while on a visit to Tripoli.

"We have a message for those who are still fighting for Gaddafi that the game is over; you have been rejected by the people of Libya," he said.